Substrains of S. mansoni selected for varying infectivity to vector snails have been tested for their ability to induce immunity in mice. Two strains derived from the same patients and designated PRT-3 and PRC-3 induced markedly different degrees of resistance to infection after a bisexual first injection. The F-1 cross between these strains also produced high resistance, comparable to that induced by the "immunogenic" PRT-3 strain. C57BL/KsJ mice with unisexual S. mansoni infections generally were about 40% resistant to challenge infections; however, the PCR-3 strain induced no resistance after unisexual infection. Although these results were repeated several times over a 3 year period, we have not been able to consistently repeat them in the past year. This nullifies any attempt to analyze the mechanisms which may have been involved and the project is being discontinued. There is some indication that the water injected with the schistosome cercariae may itself have some effect on the resistance of mice and we will reexamine this point before discontinuing the project.